Time scales and spatial patterns of passive ocean-atmosphere decay modes

Abstract.
The decay characteristics of a mixed layer ocean passively coupled to an atmospheric model
are important to the response of the climate system to stochastic or external forcing. We address
here two salient features of such decay: the scale dependence of sea surface temperature anomaly
(SSTA) decay timescales and the spatial inhomogeneities of SSTA decay modes. As expected,
decay timescales increase with the spatial extent of the SSTA. Most modes decay rapidlywith
characteristic decay times of 50-100 days for a 50 m mixed layerwith the decay determined by
local surface flux adjustment. Only those modes with spatial scales approaching or larger than
the tropical basin scale exhibit decay timescales distinctively longer than the local decay, with
the decay timescale of the most slowly decaying mode of order 250-300 days in the tropics (500
days globally). Simple analytic prototypes of the spatial scale dependence and the effect of basic
state inhomogeneities, especially the impact of nonconvecting regions, elucidate these results.
Horizontal energy transport sets the transition between the fast, essentially local, decay timescales
and the slower decay at larger spatial scales; within the Tropics, efficient wave dynamics account
for the small number of slowly-decaying modes. Inhomogeneities in the basic state climate, such
as the presence or absence of mean tropical deep convection, strongly impact large-scale SSTA
decay characteristics. For nonconvecting regions, SSTA decay is slow because evaporation is
limited by relatively slow moisture divergence. The separation of convecting and nonconvecting
region decay times and the closeness of the slower nonconvecting region decay timescale to the
most slowly-decaying modes cause a blending of properties between local nonconvecting modes
and the large-scale modes, resulting in strong spatial inhomogeneity in the slow decay modes.

Acknowledgments.
We thank John Chiang for use of the CCM3 mixed layer simulations. We
also thank Tom Farrar for valuable discussion and comments on the manuscript.
This work was supported in part by NOAA grants NA04OAR4310013 and NA05OAR4311134
and NSF grant ATM-0082529. BRL further acknowledges partial support by John
and NOAA grant NA03OAR4310066.